Service manual AlphaCom XL transceiver Rev. 1.4 10-08-2011 Blz. 69/74
NAT
In computer networking, the process of network address translation (NAT, also known as network
masquerading or IP-masquerading) involves re-writing the source and/or destination addresses of IP
Packets as they pass through a router or firewall. Most systems using NAT do so in order to enable multiple
hosts on a private network to access the Internet using a single public IP address (see gateway).
PORT
In the TCP and UDP protocols used in computer networking, a port is a special number present in the
header of a data packet. Ports are typically used to map data to a particular process running on a computer.
In the case of the PSTN2 IP, the port number can be used to route the primary and secondary transceiver
messages. The difference made by the port number defines the origin of the message.
In both TCP and UDP, each packet header will specify a source port and a destination port, each of which is
a 16-bit unsigned integer (i.e. ranging from 0 to 65535), as well as specifying the source and destination
network addresses (IP-numbers) among other things. The ALPHACOM XL and RC-4001 IP receiver card
are set to a particular port to send and receive data, meaning that it will listen for incoming packets whose
destination port matches that port number, and/or send outgoing packets whose source port is set to that port
number.
In theory, any number between 1 and 65535 can be programmed. However, much used programs mostly use
port numbers under 1024, so programming a low port number is not advised. A short list of reserved port
numbers can be found in appendix C.
ROUTER
Many Internet connections use a modem with an integrated router. This router often uses NAT. A router can
be seen as an intelligent switch,. which transports data packages from the internal network to the external
Internet and sends (routes) incoming data packages to the correct IP address.
SOCKET
The combination IP address + port number is called a SOCKET.
SWITCH / HUB
A switch or a hub is used to connect devices to a network. The difference between the two is the provided
intelligence. A hub simply sends out any received data package to the other available ports. E.g., with a 4
port hub, a received data package from the device connected to port 1, is sent to ports 2, 3 and 4.
A switch is more intelligent, it transmits a received message to all available ports, but remembers where a
reply comes from. The next time a message is sent to the same IP address, the switch routes the message
directly to the corresponding port. This greatly reduces network traffic, making the network faster and more
reliable.
TCP
TCP is one of the core protocols used on the Internet. The abbreviation TCP stands for Transmission Control
Protocol and is an international agreement on how data communication is set up.
TCP is placed above the Internet Protocol and is used to reliably transmit data packages when a connection
is established. Applications send streams of octets (8- bit bytes) to TCP for delivery through the network,
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